Our hypothesis is that gut microbiome profiles are associated with risk of colorectal cancer. Altered gut microbiome may promote colorectal carcinogenesis by inducing chronic inflammation. Also, gut microbiota may be related to CRC risk by metabolizing food components, such as carcinogenic heterocyclic amines. However, past studies of microbial flora and colorectal cancer are limited to animal models and small human studies, both considering only a selected number of culturable microbial species. To our knowledge, no epidemiologic study has comprehensively evaluated the relationship of the human gut microbiome to risk of colorectal cancer using gene sequence-based approach. We propose an innovative study to comprehensively examine the relationship between gut microbiome profile and colorectal cancer risk in a case-control study (69 cases and 114 controls), with previously collected and lyophilized fecal biospecimens obtained before initiation of any treatment, in study samples collected by NCI with associated demographic and clinical information (Schiffman MH et al, Cancer Research, 1989). We will comprehensively survey common gut microbial species (including non-culturables) from fecal biospecimens by sequencing of the 16S rRNA microbial genes. We will 1) define the normal gut microbiome in the control group, and 2) examine if the gut microbiome differs between colorectal cancer cases and controls. As a secondary aim, we will evaluate intra-individual variability (temporal stability of gut microbiome profile collected at study recruitment and after 6 months in a subset of 20 randomly selected individuals from the control group. This project is timely and highly relevant to the goals of the NIH Roadmap and the NIH Human Microbiome Project (HMP). Results from this first comprehensive evaluation of the gut microbiome will identify colorectal cancer-related gut bacterial profiles and improve our understanding of the etiology of this disease. Understanding the temporal stability of the microbiome profile in this R03 grant will lay the foundation for subsequent R01 applications, proposing large-scale prospective evaluation with fecal biospecimen collection. Considering the significant public health burden of colorectal cancer as the second leading cause of cancer death in the US, and the potential importance of the microbiome in its causation, there is great need for the proposed R03 study.